Transcutaneous transmission of energy from an external transmitter to an internal receiver is known in the prior art. Pacemakers and other types of medical devices that are implanted and require electrical energy from a battery to operate typically rely upon this type of system for recharging the battery. Although the external transmitter may be coupled to the internal receiver by a radio frequency signal, lower frequency electromagnetic coupling is generally more efficient.
To energize an implanted medical device with electromagnetically coupled power, an external transmitter coil comprising a plurality of coils of a conductor wound on a core is energized by a source of alternating electrical current. The flow of electrical current in the external transmitter coil induces a corresponding electrical current in the windings of an internal receiver coil. This electrical current can be applied to recharge the battery used by a device, or alternatively, can be employed to directly energize the implanted medical device.
Optimum transcutaneous energy transfer efficiency is achieved when the external transmitter coil is disposed on the surface of a patient's skin, directly opposite the internal receiver coil, with a minimum separation distance between the external transmitter and internal receiver coils. Depending upon the design of the external transmitter and internal receiver coils, several other factors can adversely affect the efficiency of their transcutaneous electromagnetic coupling. If the external transmitter and internal receiver coils are wound on cores, a misalignment of the pole faces of the external transmitter and internal receiver coils will reduce the coupling efficiency. Ideally, the axes of the internal receiver and the external transmitter coils should be aligned, so that electromagnetic field produced by the external transmitter coil will be concentrated in the core of the receiving coil. Any misalignment of the axes will reduce the efficiency with which energy is transferred between the two coils. However, since the internal receiver coil is inside the dermal layer of the patient's body, it is not visible. Furthermore, the internal receiver coil can shift relative to its original implanted location, so that any dye markings applied to the skin of the patient to show the original location of the internal receiver coil may become inaccurate and fail to properly indicate the position in which the external transmitter coil should be applied.
In the prior art, several techniques not relying upon external markings have been employed to ensure the proper positioning of an external transmitter coil relative to an internal receiver coil, for coils wound on pot-type cores. One technique uses an external transmitter pot core that is concave in shape, and an internal receiver pot core that has a convex shape. The convex shape of the internal receiver core disposed beneath a patient's skin creates a small bump on the epidermis. The external transmitter coil is placed over the bump and its concave shape enables it to be positioned to fit the contours of the convex bump produced by the internal receiver coil core. In this way, the external transmitter and internal receiver coils are positioned directly opposite each other so that efficient transfer of electrical power may occur.
Another technique used in the prior art employs rare earth magnets to position and support a cochlear implant. ("The Use of Rare-Earth Magnet Couplers in Cochlear Implants," K. Dormer et al., The Laryngoscope, Vol. 91, November 1981.) In this case, a cochlear stimulus signal is magnetically induced in the cochlear implant from an external coil. To support the external transmitter coil in alignment with the receiving coil of the implant, a SmCo.sub.5 disc is encapsulated in the stem of a pot core used for the internal receiver coil, and a similar SmCo.sub.5 disc of opposite magnetic polarity is included on the stem of the pot core of the external transmitter coil. The magnetic attraction between the rare earth magnetic discs tend to support and position the external transmitter coil opposite the internal receiver coil of the implant. However, this technique does not align the two coils other than along their central axes. The technique would thus not be applicable for aligning the pole faces of an external transmitter and an internal receiver having C-shaped cores, unless a pair of the rare earth magnets were used in both the external transmitter and internal receiver coils.
None of the prior art techniques enable a person to easily determine in real time the relative separation distance and alignment of the external transmitter and internal receiver coils. Significantly, frequent repositioning of the external transmitter coil may be required when an implanted medical device is energized directly by an induced electromagnetic current. A long felt need in the medical industry therefore exists for a system to indicate the position of an external transmitter coil relative to an internal receiver coil, so that a medical practitioner/patient can determine where to position the external transmitter coil to achieve optimal electromagnetic coupling to an internal receiver coil.